lkml.org 
[lkml]   [2013]   [Jul]   [19]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
/
Date
From
SubjectRe: [PATCH] hugepage: allow parallelization of the hugepage fault path
On Thu, Jul 18, 2013 at 05:42:35PM +0900, Joonsoo Kim wrote:
> On Wed, Jul 17, 2013 at 12:50:25PM -0700, Davidlohr Bueso wrote:
> > From: David Gibson <david@gibson.dropbear.id.au>
> >
> > At present, the page fault path for hugepages is serialized by a
> > single mutex. This is used to avoid spurious out-of-memory conditions
> > when the hugepage pool is fully utilized (two processes or threads can
> > race to instantiate the same mapping with the last hugepage from the
> > pool, the race loser returning VM_FAULT_OOM). This problem is
> > specific to hugepages, because it is normal to want to use every
> > single hugepage in the system - with normal pages we simply assume
> > there will always be a few spare pages which can be used temporarily
> > until the race is resolved.
> >
> > Unfortunately this serialization also means that clearing of hugepages
> > cannot be parallelized across multiple CPUs, which can lead to very
> > long process startup times when using large numbers of hugepages.
> >
> > This patch improves the situation by replacing the single mutex with a
> > table of mutexes, selected based on a hash, which allows us to know
> > which page in the file we're instantiating. For shared mappings, the
> > hash key is selected based on the address space and file offset being faulted.
> > Similarly, for private mappings, the mm and virtual address are used.
> >
>
> Hello.
>
> With this table mutex, we cannot protect region tracking structure.
> See below comment.
>
> /*
> * Region tracking -- allows tracking of reservations and instantiated pages
> * across the pages in a mapping.
> *
> * The region data structures are protected by a combination of the mmap_sem
> * and the hugetlb_instantion_mutex. To access or modify a region the caller
> * must either hold the mmap_sem for write, or the mmap_sem for read and
> * the hugetlb_instantiation mutex:
> *
> * down_write(&mm->mmap_sem);
> * or
> * down_read(&mm->mmap_sem);
> * mutex_lock(&hugetlb_instantiation_mutex);
> */

Ugh. Who the hell added that. I guess you'll need to split of
another mutex for that purpose, afaict there should be no interaction
with the actual, intended purpose of the instantiation mutex.

--
David Gibson | I'll have my music baroque, and my code
david AT gibson.dropbear.id.au | minimalist, thank you. NOT _the_ _other_
| _way_ _around_!
http://www.ozlabs.org/~dgibson
[unhandled content-type:application/pgp-signature]
\
 
 \ /
  Last update: 2013-07-19 10:01    [W:0.077 / U:6.264 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site